Sensitivity of Krasnodar Venturia inaequalis Populations to the Sterol Demethylation Inhibitor Difenoconazole.

Difenoconazole sensitivity was assessed in three populations of the apple scab agent Venturia inaequalis of the Krasnodar region. One of the populations was fungicide naïve population; its sample was collected in natural habitats of the eastern crabapple Malus orientalis in foothills of the Severskii raion. The two other populations were from commercial orchards of the apple variety Reinette Simirenko (the Krasnoarmeiskii and Dinskoi raions), where fungicide treatments with various agents, including those with difenoconazole as an active ingredient, were performed on a regular basis. Single-spore V. inaequalis isolates were isolated from fresh leaves with signs of the disease or fallen leaves with fungal fruiting bodies. The median effective concentration (EC) was defined as the concentration that halves the colony growth and was estimated in a series of in vitro experiments with 120 isolates. Difenoconazole (Score EC, 250 mg/L, Syngenta) was used at six concentrations: 0.005, 0.01, 0.025, 0.05, 0.5, and 1 mg a.i./L. Mean EC values were 0.0078 mg a.i./L in the natural population and 0.12 and 0.25 mg a.i./L in the orchard populations. Fungicide sensitivity was lower in both of the orchard populations; their resistance factors were estimated at 16 and 32. The proportion of sensitive and low-sensitive isolates differed between the two orchard populations. A discriminatory dose, or single-assessment concentration (SAC), of 0.01 mg a.i./L was proposed to simplify the laboratory monitoring of the difenoconazole sensitivity in V. inaequalis by using a test for relative growth (RG) of the mycelium. Comparable results were obtained with EC and RG at the discriminatory dose. The portion of isolates with RGs exceeding the threshold (RG > 70%) was 97% in one of the orchard populations. The results indicate that difenoconazole resistance develops in V. inaequalis populations from commercial orchards of the Krasnodar region.